There are applications wherein backward propagation of a wave signal is prevented such as to suppress cross-talk and interferences. Basically, if light can travel from A to B, it can also travel from B to A. Breaking this law can be achieved thanks to an optical isolator or optical diode, i.e. an optical component which allows the transmission of light in only one direction. Such devices mostly rely on the Faraday effect (a magneto-optic effect). However, they are generally polarization dependent. Should the polarization problem be overcome, it remains that such devices are not suited for integration and can thus not be contemplated for applications such as optical signal processing on-chip and integrated optics in general.
The following patent documents provide interesting details as to the background art in the field:
U.S. Pat. No. 5,574,595: discloses a method for polarization independent optical isolation based on magneto-optic effect;
U.S. Pat. No. 5,267,078: proposes a system comprising polarization splitters for selective isolation of different polarization states of light;
U.S. Pat. No. 7,113,676 is directed to a planar waveguide optical isolator in thin silicon-on-insulator (SOI) structure; and
U.S. Pat. No. 5,764,681 suggests a directional control method and apparatus for ring laser using separate mirrors.
As a matter of fact, a solution which is both cost-efficient and all the more compatible with CMOS technology is hard to obtain.
Next, in addition to patent literature, a number of publications are devoted to the subject, for instance:
[1] T. R. Zaman, X. Guo and R. J. Ram: “Proposal for a Polarization-Independent Integrated Optical Circulator”. IEEE Photonics Technology Letters, 18(12), 1359-1361, (2006).
[2] N. Bahlmann, M. Lohmeyer, A. Zhuromskii, H. Doetsch and P. Hertel: “Nonreciprocal coupled waveguides for integrated optical isolators and circulators for TM-modes”. Optics Communications, 161(4-6), 330-337, (1999).
[3] T. Zaman, X. Guo and R. J. Ram, “Integrated Optical Circulator in InP”, Conference on Lasers and Electro-Optics CLEO, 2, 1321-1323, (2005).
[4] J. Y. Lee, X. Luo and A. W. Poon, Reciprocal transmissions and asymmetric modal distributions in waveguide-coupled spiral-shaped microdisk resonators, Optics Express 15(22), 14650-14666, (2007).
[5] Z. Yu and S. Fan, “Complete optical isolation created by indirect interband photonic transitions”. Nature photonics 3, 91-94, (2009).
[6] M. T. Hill, H. J. S. Dorren, T. de Vries, J. M. Leijtens, J. H. den Besten, B. Smalbrugge, Y.-S. Oei, H. Binsma, G.-D. Khoe and M. K. Smit. “All-optical memory based on coupled microring lasers”. Nature 432, 206-209 (2004).
In particular, the solution in Ref. [1] is manifestly not compatible with CMOS/silicon photonics. The solution proposed in Ref. [5] requires a variation of the index of refraction (i.e. of the material) and a successive filter.